In electrophotography, generally, a surface of a latent image bearing member is charged using a process such as corona discharge followed by exposure using laser to form an electrostatic latent image. The resulting electrostatic latent image is developed by a toner to form a toner image. An image with high quality can be obtained by transferring the resulting toner image on a recording medium. Typically, toner particles (toner base particles) with an average particle diameter of from 5 μm to 10 μm, produced by mixing a binder resin such as a thermoplastic resin with toner components such as a colorant, a charge control agent, a release agent, and a magnetic material and then passing the mixture through the steps of kneading, pulverizing, and classifying, are used for the toner applied to such electrophotography. In addition, in order to provide flowability or appropriate charging performance to the toner or to facilitate cleaning of the toner from surfaces of photoconductor drums, silica and/or inorganic fine particles such as those of titanium oxide are externally added to the toner base particles.
A two-component developing system using a toner and a carrier such as an iron powder, and a magnetic single-component developing system using only a toner containing toner particles containing a magnetic powder inside without using a carrier, are known as dry developing processes in various electrophotographic systems that are currently in practical use. The toner containing toner particles containing a magnetic powder (hereinafter also referred to as a magnetic toner) used in the magnetic single-component developing system has advantages such as low cost and excellent durability.
Furthermore, from the standpoint of energy saving, toners containing toner particles each having a core-shell structure in which a toner core particle using a low-temperature binder resin is coated with a shell material formed of a resin having a higher glass transition point (Tg) than the Tg of the binder resin contained in the toner core particle have been used conventionally to improve low-temperature fixability, storage stability at high temperatures and antiblocking properties.
As for toner which includes toner particles with such a core-shell structure, a toner which includes toner particles with a core-shell structure, composed of toner core particles containing a polyester resin or a resin where a polyester resin and a vinyl resin are bound and a shell layer consisting of a shell material containing a copolymer between styrene and a (meth)acrylic monomer containing a polyalkylene oxide unit, has been proposed. The toner particles with this core-shell structure are formed by coating a surface of toner core particles with resin fine particles dispersed in an aqueous medium in the presence of an organic solvent such as ethyl acetate.
However, in the shell layers of the toner particles in the toner, since contact sites of the resin fine particles themselves have been dissolved by the organic solvent, there remains almost no void between the resin fine particles and uniform films are formed in a condition that the shape of resin fine particles remains. Therefore, when forming images using the toner, the shell layer may be resistant to break during fixing images on recording media even when a pressure is applied to the toner particles in the toner. In cases where the shell layer cannot be easily broken, it is difficult to appropriately fix the toner on recording media.
Furthermore, in the case where the above-mentioned toner particles are used as magnetic toner particles by incorporating a magnetic powder into toner core particles, the toner particles are sometimes difficult to be charged at a desired charge amount in an environment of high temperature and high humidity, depending on the state of the shell layers. Therefore, in the case where an image is formed by using a toner containing magnetic toner particles each having a core-shell structure obtained by the above-mentioned method in an environment of ordinary temperature and ordinary humidity or an environment of high temperature and high humidity over a long time period, it is sometimes difficult to form an image having a desired image density. The defects related to image formation over a long time period are more significant in an environment of high temperature and high humidity.